The UI in general feels like very old legacy code (at least 2 or 3 years ago). It also runs extremely slow compared with other providers out there now days - which it needs work done! You have two choices here- either use another provider OR write your own solution using one off resources available online as this looks outdated even when comparing with services offered by others who are still actively engaged at doing such things today! My team has successfully migrated most parts our customer database into mailchimp since I started working full time over 5 months back - but we were not able get some custom fields populated properly along with tags integration part in particular so was facing issues sending mails after migration!! But overall everything works fine otherwise. I like how easy it is to use. The reporting is very detailed so we can see exactly what our emails are doing. It's a great tool for any size business. There is nothing i dislike about this product. I would recommend trying it out. It does take some time to get used to but once you do it's easy to use. We have been able to send more targeted emails which has helped us grow our list.